Bump to package substrate solder joint

ABSTRACT

A method of assembling a flipchip semiconductor package, includes placing a semiconductor die having circuitry electrically coupled to bond pads with bumps having solder paste thereon onto bonding features of a package substrate. Arc welding is used using an arc welding apparatus including a biased electrode having a tip spaced apart from the solder paste, wherein electrical current generated by the arc welding melts the solder paste to provide a solder connection.

FIELD

This Disclosure relates to a flipchip semiconductor package.

BACKGROUND

A flipchip semiconductor package generally includes a solder connectionbetween bumps on bond pads of a semiconductor die and bonding feature ona package substrate, such as leads of a leadframe. The bumps generallycomprise solder balls or copper pillars. Upon placement of thesemiconductor die on the package substrate, there is generally a solderreflow step performed at a temperature of at least 220° C. to form thesolder joint.

SUMMARY

This Summary is provided to introduce a brief selection of disclosedconcepts in a simplified form that are further described below in theDetailed Description including the drawings provided. This Summary isnot intended to limit the claimed subject matter's scope.

Disclosed aspects recognize there can be reliability issues on flipchipdevices due to non-wet or an insufficient solder joint connectioninduced by solder flux dry up and solder inactivation during flipchipdie attach. This problem can be attributed to a long window of timebetween die attach and the solder reflow process.

Disclosed aspects include a method of assembling a flipchipsemiconductor package, comprising placing a semiconductor die havingcircuitry electrically coupled to bond pads having a bump thereongenerally comprising a solder ball or copper pillar, with solder pastethereon, onto bonding features of a package substrate. Arc welding isutilized using an arc welding apparatus comprising an arm and a biasedelectrode having a tip spaced apart from the solder paste, whereinelectrical current generated by the arc welding melts the solder toprovide a solder connection.

BRIEF DESCRIPTION OF DRAWINGS

Reference will now be made to the accompanying drawings, which are notnecessarily drawn to scale, wherein:

FIGS. 1A-1C are cross-sectional depictions showing steps in an examplearc welding method for forming solder connections between asemiconductor die and bonding features of a package substrate, accordingto an example aspect.

FIG. 2 depicts an optional orc welding apparatus for simultaneouslyforming a plurality of solder connections between a semiconductor dieand bonding features of a package substrate, according to an exampleaspect.

FIG. 3 shows steps in an example arc welding method of assembling aflipchip semiconductor device having solder joints, according to anexample aspect.

DETAILED DESCRIPTION

Example aspects are described with reference to the drawings, whereinlike reference numerals are used to designate similar or equivalentelements. Illustrated ordering of acts or events should not beconsidered as limiting, as some acts or events may occur in differentorder and/or concurrently with other acts or events. Furthermore, someillustrated acts or events may not be required to implement amethodology in accordance with this Disclosure.

FIGS. 1A-1C are cross-sectional depictions showing steps in an examplemethod for forming solder connections between a semiconductor die havingbumps on its bond pads and bonding features of a package substrate,according to an example aspect. FIG. 1A shows a semiconductor die 120having bond pads 121 with bumps 122 with solder paste 126 thereon afterflipchip die attachment (shown by an arrow) to leads 131 (providingbonding features that may be also referred to being contact pads) of thepackage substrate shown as a leadframe 130 having optional solder flux132 on the leads 131. All bond pads are positioned along a periphery ofthe semiconductor die and leads 131 along the periphery of the leadframe130 or other package substrate to enable the arc welding process tooccur.

Although not shown, there is generally under bump metallization (UBM)under the bump 122. Solder flux generally comprises a base soldermaterial and an activator which is the chemical that promotes betterwetting of the solder by removing oxides from the solder metal material.The package substrate can also comprise a printed circuit board (PCB),Roughened/Rough Lead Frame (RLF), Molded Interconnect Substrate (MIS),Etched Lead Frame, or a Pre-plated Lead Frame, that provides the bondingfeatures.

FIG. 1B shows the arm 142 connected to a torch electrode tip (tip) 141of an arc welding apparatus 150 after being placed in proximity to thesolder paste 126. The arc welding apparatus 150 generally includes amotor for moving the arm 142 to move the arm for moving the tip 141. Thearc welding apparatus generally can sense the distance between the tipand the solder paste 126. The arc welding apparatus generally has aposition sensing device which may be similar to that used inconventional bonding process where there is typically a camera to locatethe bond pads 121. With the aid of the camera, the arm 142 is moved sothat the tip 141 is generally positioned about 0.05 mm to 0.2 mm fromthe solder paste 126 and at an angle of about 30° relative to the topsurface of the package substrate for best results. Positioning the tip141 less than 0.05 mm or greater than 0.2 mm from the solder paste canlead in some cases to partial, incomplete or less satisfactory reflow ofsolder paste 126.

There is generally no direct current return path provided for the arcwelding apparatus, where the leadframe or other package substrate caninstead be grounded indirectly by the holder of the package substratewhich is itself grounded for the current to be terminated and aconnection to the power supply of the arc welding apparatus to provide aground for the power supply, and to also serve as a protection for thesemiconductor die 120.

Arc welding as used herein takes on its conventional definition being awelding process that is used to join metal to metal by using electricity(displacement current) to create enough heat to melt metal, and themelted metals, when cool, result in a binding together of the respectivemetals. A welding power supply is used to create an electric arc betweena metal stick (tip functioning as an electrode) and the base material tomelt the metals at the point of contact. Arc welders can use eitherdirect (DC) or alternating (AC) current.

FIG. 1C shows the device after high current induced melting of thesolder paste to form solder wetting of the solder paste now shown as 126a. A package substrate such as a leadframe sheet having a plurality ofleadframes is generally processed together. Moreover, a moldencapsulation step generally follows in the case of a molded package.

FIG. 2 depicts an optional arc welding apparatus 200 having a pluralityof arms 142 a, 142 b, 142 c and 142 d with electrodes having respectivetips 201, 202, 203, 204 along with a power supply 250 having multiplechannels for simultaneously forming a plurality of solder connectionsbetween bumps on bond pads of a semiconductor die and bonding featuresof a package substrate, according to an example aspect. In the depictionshown, tips 201 and 203 each provide solder processing for a solderconnection in the plane of the page, tip 202 provides processing forsolder connections coming out of the page, and tip 204 providesprocessing for solder connections going into the page.

FIG. 3 shows steps in an example method 300 of assembling a flipchipsemiconductor device, according to an example aspect. Step 301 comprisesplacing a semiconductor die having circuitry electrically coupled tobond pads having bumps with solder paste thereon onto bonding featuresof a package substrate on the bonding features. Step 302 comprises arcwelding using a biased electrode having a tip spaced apart from thesolder paste. Current is induced by the electrode, positioned at anangle generally from 15 to 45°, such as −30° from the package substrate,onto the solder paste. Electrical displacement current is generated bythe arc welding melts the solder paste for best results. Positioning theelectrode at an angle less than 15 or greater than 45 (degrees) from thepackage substrate can lead in some cases to partial, incomplete or lesssatisfactory reflow of solder paste 126. Each solder contact can utilizea separate applying current step, or using a suitable multi-channel arcwelding apparatus as described above in FIG. 2 shown as 200 where aplurality of solder contacts can be formed simultaneously.

The bumps can include pillars or sider balls. The method generallyexcludes a solder reflow step. The method can include flux washing afterthe application of the electrical current. As noted above, the arcwelding apparatus used for the method can include a plurality of biasedelectrodes, wherein the arc welding simultaneously forms a plurality ofthe solder connections.

The package substrate can comprise a leadframe, where the bondingfeatures comprise leads. The tip can be positioned at an angle from 15to 45° relative to a normal drawn from a top surface of the packagesubstrate. The bond pads are generally all positioned along a peripheryof the semiconductor die, such as being in a rectangular or circularshape. The electrical current can be in a range from 60 mA to 200 mA andcan be applied for a time between 0.5 msec-and 1.3 msec solder paste forbest results. Using electrical current outside the range of 60 mA to 200mA and/or applying the electrical current for a time outside the rangeof 0.5 msec and 1.3 msec can lead in some cases to partial, incompleteor less satisfactory reflow of solder paste 126.

1. A method of assembling a flipchip semiconductor package, comprising:placing a semiconductor die having circuitry electrically coupled tobond pads having bumps with solder paste thereon onto bonding featuresof a package substrate; and arc welding using a biased electrode havinga tip spaced apart from the solder paste, wherein electrical currentgenerated by the arc welding melts the solder paste to provide a solderconnection.
 2. The method of claim 1, wherein the bumps comprise solderballs or pillars.
 3. The method of claim 1, wherein the method excludesa solder reflow step.
 4. The method of claim 1, further comprising fluxwashing after applying the electrical current.
 5. The method of claim 1,wherein an arc welding apparatus used for the method includes aplurality of the biased electrodes, and wherein the arc weldingsimultaneously forms a plurality of the solder connections.
 6. Themethod of claim 1, wherein the package substrate comprises a leadframe,and wherein the bonding features comprise leads.
 7. The method of claim1, wherein the tip is positioned at an angle from 15° to 45° relative toa normal drawn from a top surface of the package substrate.
 8. Themethod of claim 1, wherein the bond pads are all positioned along aperiphery of the semiconductor die.
 9. The method of claim 1, whereinthe electrical current is in a range from 60 mA to 200 mA.
 10. Themethod of claim 1, wherein the electrical current is applied for a timebetween 0.5 msec and 1.3 msec.
 11. The method of claim 1, wherein an arcwelding apparatus used for the method includes a substrate holder havingthe package substrate thereon that is grounded, and wherein the packagesubstrate is grounded by the substrate holder.
 12. The method of claim1, wherein there is solder flux on the bonding features.
 13. A method ofassembling a semiconductor package, comprising: placing a semiconductordie having circuitry electrically coupled to electrical contacts withsolder paste thereon onto bonding features of a package substrate; andarc welding using a biased electrode having a tip spaced apart from thesolder paste, wherein electrical current generated by the arc weldingmelts the solder paste to provide a solder connection.
 14. The method ofclaim 13, wherein the package substrate comprises a leadframe, andwherein the bonding features comprise leads.
 15. The method of claim 13,wherein the tip is positioned at an angle from 15° to 45° relative to anormal drawn from a top surface of the package substrate.
 16. The methodof claim 13, wherein the bond pads are all positioned along a peripheryof the semiconductor die.
 17. The method of claim 13, wherein theelectrical current is in a range from 60 mA to 200 mA.
 18. The method ofclaim 13, wherein the electrical current is applied for a time between0.5 msec and 1.3 msec.
 19. The method of claim 13, wherein the bumpscomprise pillars.
 20. The method of claim 13, wherein the bumps comprisesolder balls.
 21. The method of claim 13, wherein the method excludes asolder reflow step.
 22. A method of assembling a semiconductor package,comprising: placing a semiconductor die having circuitry electricallycoupled to electrical contacts with solder paste thereon onto bondingfeatures of a package substrate; and generating an arc using a biasedelectrode having a tip spaced apart from the solder paste to melt thesolder paste to provide a solder connection.